High temperature thermal processes, for example, the generation of steam for the production of electricity in power plants utilizing fossil fuels and biomass or the incineration of domestic waste, often create environmentally harmful by-products. These, among others, include nitrogen oxides (NOx—refers to NO and NO2) and sulfur oxides (SO2 and SO3). These compounds have to be removed from the flue gases of the high temperature thermal process before being discharged to the environment.
The standard for removing NOx from flue gases is the Selective Catalytic Reduction (SCR) process, where a reducing reagent, typically ammonia, is injected, mixed into the flue gas, and sent through a catalytic reaction chamber where a catalyst facilitates the reduction of NOx with the reducing agent to form nitrogen gas and water.
Throughout the operation of the catalyst, it becomes contaminated due to the accumulation of various substances from the flue gas on the catalyst. Most of them are responsible for the catalyst's decrease in activity, such as sodium, potassium, phosphorus and arsenic. Others, like iron, however, are known to be the main contributor for the increase of the SO2/SO3 conversion rate during the catalysts usage cycle. This type of contamination is due to chemical bonding of the compounds onto the catalyst.
In addition, the catalyst can become physically plugged with fly ash—the combustion residue from the fuel incinerated in the thermal process. During the operation of the SCR reactor the fly ash accumulates within and on top of the catalyst. The gradient of contamination is typically the highest on the flue gas inlet side and decreases towards the flue gas outlet side.
The fly ash removal technology is geared towards the physical properties of the fly ash which varies depending on the fuel type and operating conditions in the thermal process. It can be a fine powder or Large Particle Ash (LPA) (approximately 0.2 to 1 inch) and can develop into big chunky pieces (approximately 1 to 5 inches) when accumulating on the catalyst. Both types of fly ash form in the boiler and easily carry over into the SCR reactor causing catalyst pluggage which leads to flue gas maldistribution, loss of SCR performance through loss of available DeNOx potential, unacceptable NH3 slip, excessive pressure drop and catalyst erosion damage.
Fine powder fly ash can be removed using Electro Static Precipitators (ESP) which are installed upstream or downstream of the SCR reactor depending on the SCR arrangement (High Dust, Low Dust, or Tail End). The LPA, also known as popcorn ash, can be collected prior to the SCR reactor by means of LPA screens, which are typically located between the economizer outlet and SCR inlet.
Depending on the source and makeup of the fuel incinerated, the components of the fly ash produced vary considerably. Fly ash typically includes varying amounts of silica (silicon dioxide, SiO2) (both amorphous and crystalline), aluminum oxide (Al2O3), iron oxide (Fe2O3), lime (calcium oxide, CaO), and other types of calcium material (various calcium salts such as calcium carbonate and calcium sulfate).
Despite the above mentioned technologies, the fly ash removal may not be sufficient to protect the catalyst from pluggage which leads to premature loss of SCR performance. Loose powder can plug the channels of honeycomb-type catalyst with individual channels being partially or fully inaccessible for the flue gas. Chunky pieces can deposit on top of catalyst modules and block the flue gas passage through honeycomb, plate or corrugated catalyst. Popcorn ash travels through the channels of honeycomb style catalysts and deposits at a location where it can get “wedged” between the channel walls providing an environment for loose powder ash to accumulate and plug the channel. The result can be a catalyst with a pluggage rate ranging from approximately 10% to 100% rendering the catalyst with a decreased NOx removal efficiency.
In some applications the physical contamination outweighs the chemical contamination. The result is a decrease of the operation time and a pre-mature exchange of the catalyst. Pluggage removal in-situ (installed in the SCR reactor), ex-situ and on site (removed from the SCR reactor and treated on premise), or ex-situ and off site (removed from the SCR reactor and treated at a regeneration facility) could extend the operation time of the catalyst.
Substrates (e.g., catalytic converters) that contain calcium material deposited thereon (e.g., particularly calcium-containing fly ash material) have been found particularly difficult to clean, and ultimately, rejuvenate. Additional methods of treating substrates and rejuvenating catalytic converters are highly desired.
Accordingly, there remains a need, at least, for a more efficient method to remove the calcium-containing fly ash, and to open and unplug catalyst channels to provide a fly ash-free (or nearly-fly ash-free) catalyst prior to a wet-chemical rejuvenation or regeneration process.